Early multistation locks obtained rotational control of the plug by means of stop washers that rotated with the plug and engaged stops on the housing so as to limit angular movement of the plug. In one application a stop washer limits movement of the plug in only one direction from the lock position to a single, unlock station circumferentially spaced therefrom, or alternatively allows rotational movement in opposite directions from a lock position to a maximum of two stations circumferentially spaced from the lock position. This stop washer technique for rotational control was used in conjunction with cylinder locks that employ a key-actuated plug rotatably mounted within a cylindrical housing. The plug typically retains a plurality of tumblers that are spring-biased into engagement with an accommodating spline on the inner surface of the housing to prevent rotation of the plug within the housing. Insertion of a properly bitted key into the plug draws the tumblers into shear positions and allows the plug to be rotated to other positions as controlled by the stop washers. However, this approach had the shortcoming that the direction of the plug rotation and therefore access to either operational station was at the discretion of the user possessing the single proper key.
More recently, a multistation lock was developed, U.S. Pat. Nos. 4,057,987 and 3,973,421, which used one or more wafer tumblers to control movement of a lock to different stations. Each station was controlled by a separate key which could not operate stations other than its own. With such an approach, however, one or more wafer tumblers are committed to the positional determination and so there is less space and fewer tumblers remaining to provide the locking combination. In addition, prior art locks have been quite susceptible to riffing and other techniques of illegal lock operation.
Locks of both wafer and pin type are subject to illicit manipulation by keys other than those proper to the lock being manipulated by a process known as riffing. Riffing consists of inserting other than the proper key in the keyway, applying light torque in the opening direction, and while maintaining torque moving the key axially in the plug. The action of the key profile in most cases causes the wafer to come to shear at various times during the action. As the wafer comes to shear the torque on the plug forces the edge of the wafer under the edge of the housing or shell spline. Continuing light torque maintains the wafer in that position while the repeated axial movement brings other wafers to shear, where they are in turn trapped. The trapping of each wafer provides a slight additional amount of angular motion of the plug, thus aiding the process.
Another common condition that exists in wafer locks is that of freedom of the key to be moved in a tilting manner in such a way as to lift the wafers in the unlocking direction to varied degrees. This motion is often used in conjunction with the axial movement of the key in riffing. Various means have been attempted with varying degrees of success to block such illicit action. It is particularly important that locks controlling electronic apparatus be secure from this type of tampering. Unauthorized entry into a particular station might reveal restricted data which should be available only to the station's rightful key holder. Further, such tampering could permit erasure of memory banks and provide opportunity for theft and embezzlement.